B650M Pro RS no POST after BIOS update

Try following these steps to ensure you don't have any corrupt values in your
BIOS:
https://forum.asrock.com/forum_posts.asp?TID=630&title=how-to-clear-cmos-via-battery-removal

Instructions to use flashback if that doesn't work can be found in your user manual
but here they are again for convenience:

user manual wrote: To use the USB BIOS Flashback function, Please follow the steps below. 1. Download the latest BIOS file from ASRock's website : http://www.asrock.com. 2. Copy the BIOS file to your USB flash drive. Please make sure the file system of your USB flash drive must be FAT32. 3. Extract BIOS file from the zip file. 4. Rename the file to ?œcreative.rom??and save it to the root directory of X: USB flash drive. 5. Plug the 24 pin power connector to the motherboard. Then turn on the power supply's AC switch. *There is no need to power on the system. 6. Then plug your USB drive to the USB BIOS Flashback port. 7. Press the BIOS Flashback Switch for about three seconds. Then the LED starts to blink. 8. Wait until the LED stops blinking, indicating that BIOS flashing has been completed. *If the LED light turns solid green, this means that the BIOS Flashback is not operating properly. Please make sure that you plug the USB drive to the USB BIOS Flashback port. **If the LED does not light up at all then please disconnect power from the system and remove/disconnect the CMOS battery from the motherboard for several minutes. Reconnect power and battery and try again.

Ensure the flashback worked properly as per the LED behavior listed above, if
you didn't follow the steps correctly or the flash drive you used is not compatible
for some reason the flashback can often fail to begin/complete. Sometimes it helps
to use flashback multiple times.

Good luck